68HC705SP7 datasheet(156/230 Pages) FREESCALE

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Programmable Timer

General Release Specification

MC68HC705JJ7/MC68HC705JP7 — Rev. 3.0

Programmable Timer

11.2 Introduction

The MC68HC705JJ7/MC68HC705JP7 MCU contains a 16-bit

programmable timer with an input capture function and an output

compare function as shown by the block diagram in Figure 11-1.

The basis of the capture/compare timer is a 16-bit free-running counter

which increases in count with every four internal bus clock cycles. The

counter is the timing reference for the input capture and output compare

functions. The input capture and output compare functions provide a

means to latch the times at which external events occur, to measure

input waveforms, and to generate output waveforms and timing delays.

Software can read the value in the 16-bit free-running counter at any

time without affecting the counter sequence.

The I/O registers for the input capture and output compare functions are

pairs of 8-bit registers, because of the 16-bit timer architecture used.

Each register pair contains the high and low bytes of that function.

Generally, accessing the low byte of a specific timer function allows full

control of that function; however, an access of the high byte inhibits that

specific timer function until the low byte is also accessed.

Because the counter is 16 bits long and preceded by a fixed divide-by-

four prescaler, the counter rolls over every 262,144 internal clock cycles

(every 524,288 oscillator clock cycles). Timer resolution with a 4-MHz

crystal oscillator is 2 microsecond/count.

The interrupt capability, the input capture edge, and the output compare

state are controlled by the timer control register (TCR) located at $0012,

and the status of the interrupt flags can be read from the timer status

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